1. Field of the Invention
This invention relates in general to equipment for transferring fluids. In particular, the invention relates to a product swivel and a swivel assembly adapted for transferring fluids between tankers, storage vessels and the like and one or more conduits beneath the ocean surface.
2. Description of the Prior Art
The offshore search for oil and gas has greatly expanded in recent years and progressed into deep rough waters such as the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex mooring systems for floating vessels which serve as centralized production sites for the entire field have been developed. Flexible fluid lines extend from a subsea location to the mooring site to permit the transfer of fluids between a moored vessel and a subsea location. For example, certain fluid lines may be used to convey oil and gas into the floating vessel while other fluid lines may be used to inject liquids or gases back into the field for purpose of field stimulation or storage.
Floating vessels can be moored to a single point mooring system, which permits the vessel to weather vane and rotate 360 degrees about a single mooring point. To permit the vessel to rotate and move freely without causing twisting or entanglement of the various fluid lines to which the vessel is attached, it is necessary to provide a swivel mechanism to connect the fluid lines to the mooring site. Furthermore, since a plurality of fluid lines are involved, it is necessary that such a swivel have the capability of accommodating multiple lines.
Multiple line swivels for offshore use are known in the prior art. For example, U.S. Pat. No. 4,306,741 issued to Foolen describes a swivel assembly where a plurality of swivels are mounted coaxially and have an axial radial bearing structure with axial rollers and radial rollers bearingly supporting the inner and outer rings of each swivel as well as supporting one swivel with respect to each other on top of another. The seals of the Foolen patent are radial seals which seal against the outer wall of the inner ring. The seals are disposed in radial grooves in the inner wall of the outer ring making it practically impossible to assemble the swivel unit with seals of adequate rigidity to withstand the required pressure without being extruded into the seal gap between the inner and outer swivel rings. Seal material of adequate rigidity cannot be stretched enough for the seal ring and be inserted in a radial groove in the inner ring or compressed enough for insertion into radial grooves in the outer ring.
The radial seals of the prior art do not operate efficiently under high pressure because high pressure product in the conduit between the outer ring and the inner ring tends to separate rings. In other words, the inner ring deflects inwardly and the outer ring deflects outwardly, thereby forcing the radial seals out of sealing engagement of one ring from the other ring. The radial seals of the Foolen patent, as referenced above are in axial alignment above and below the conduit chamber thereby resulting in balanced axial loading between the inner and outer rings.
The prior art bearing assemblies, though designed to transfer large radial and thrust loads, lose their concentricity as the inner and outer rings deflect away from each other upon pressurization of the conduit chamber, as discussed above, thereby creating looseness in the bearing assembly. Also, as the bearing assembly wears, additional looseness results in the bearing assembly.
Such problems are compounded when the swivels are exposed to extremely high pressures.